Don't. Simple answer. Don't until the current project is complete. Unless you have some absolutely compelling reason that means you must upgrade. If so spell it out here to what's so important in FCP7 that you'll risk your project for it.

It's never as simple to upgrade a project to a new software version as it should be. You'd probably be ok if you haven't got any complicated effects going on but it's a gamble, my experience is that whenever I've had software upgraded during a job (not just on a computer, I once had an upgrade on a big vision mixer done overnight) something has gone awry somewhere.